Solid Zirconium-Based Cross-Linking Agent and Use in Oil Field Applications

ABSTRACT

A solid zirconium cross-linking agent and use in a cross-linking composition in oil field applications such as hydraulic fracturing and plugging of permeable zones. The zirconium cross-linking agent is prepared by a process comprising contacting a zirconium complex with an alkanolamine and water at particular mole ratios of alkanolamine and water to zirconium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/986,397, filed Nov. 21, 2007 and published as U.S.Application Publication No. 2009/0131282 A1, entitled “SolidZirconium-Based Cross-Linking Agent and Use in Oil Field Applications,”which is hereby incorporated herein by reference in its entirety for allpurposes.

FIELD OF THE INVENTION

The present invention relates to solid zirconium chelates and their usein oil field applications such as hydraulic fracturing and plugging ofpermeable zones.

BACKGROUND OF THE INVENTION

The production of oil and natural gas from an underground well(subterranean formation) can be stimulated by a technique calledhydraulic fracturing, in which a viscous fluid composition (fracturingfluid) containing a suspended proppant (e.g., sand, bauxite) isintroduced into an oil or gas well via a conduit, such as tubing orcasing, at a flow rate and a pressure which create, reopen and/or extenda fracture into the oil- or gas-containing formation. The proppant iscarried into the fracture by the fluid composition and prevents closureof the formation after pressure is released. Leak-off of the fluidcomposition into the formation is limited by the fluid viscosity of thecomposition. Fluid viscosity also permits suspension of the proppant inthe composition during the fracturing operation. Cross-linking agents,such as borates, titanates or zirconates, are usually incorporated intothe fluid composition to control viscosity.

Typically, less than one third of available oil is extracted from a wellafter it has been fractured before production rates decrease to a pointat which recovery becomes uneconomical. Enhanced recovery of oil fromsuch subterranean formations frequently involves attempting to displacethe remaining crude oil with a driving fluid, e.g., gas, water, brine,steam, polymer solution, foam, or micellar solution. Ideally, suchtechniques (commonly called flooding techniques) provide a bank of oilof substantial depth being driven into a producing well; however, inpractice this is frequently not the case. Oil-bearing strata are usuallyheterogeneous, some parts of them being more permeable than others. As aconsequence, channeling frequently occurs, so that the driving fluidflows preferentially through permeable zones depleted of oil (so-called“thief zones”) rather than through those parts of the strata whichcontain sufficient oil to make oil-recovery operations profitable.

Difficulties in oil recovery due to thief zones may be corrected byinjecting an aqueous solution of an organic polymer and a cross-linkingagent into a subterranean formation under conditions where the polymerwill be cross-linked to produce a gel, thus reducing permeability of thesubterranean formation to the driving fluid (gas, water, etc.).Polysaccharide- or partially hydrolyzed polyacrylamide-based fluidscross-linked with certain aluminum, titanium, zirconium, and boron basedcompounds are used in these enhanced oil recovery applications.Cross-linked fluids or gels, whether for fracturing a subterraneanformation or for reducing permeability of zones in subterraneanformation, are now being used in hotter and deeper wells under a varietyof temperature and pH conditions. In these operations the rate ofcross-linking is critical to the successful generation of viscosity.

Oil field service companies are currently using zirconium or titaniumbased cross-linkers to generate viscosity in polysaccharide-based fluidsuseful in hydraulic fracturing, completion and enhanced oil recoveryapplications. Commercially available, zirconium-based cross-linkerscontaining an alkanolamine or a hydroxyalkylated ethylenediamine as achelating ligand cross-link in the desired time and generate andmaintain significant viscosity.

For example, U.S. Pat. No. 4,883,605 discloses a water-soluble zirconiumchelate formed from a tetraalkyl zirconate andhydroxyethyl-tris-(2-hydroxypropyl) ethylenediamine, and the use of thechelate as a cross-linking agent in hydraulic fracturing fluids and ingels that are used for selectively plugging permeable zones insubterranean formations or for plugging subterranean leaks. Co-pendingpatent application, “Stable Solutions of Zirconium HydroxyalkylethyleneDiamine Complex and Use in Oil Field Applications”, U.S. patentapplication Ser. No. 11/643,513, filed Dec. 21, 2006, published at US2008/0149341 A1, discloses a related complex having a 1:1 molar ratio ofzirconium and N,N,N′,N′-tetrakis-(2-hydroxypropyl)-ethylenediamine.

These and other existing titanium or zirconium-based cross-linkers suchas TYZOR TE organic titanate or TYZOR TEAZ organic zirconate are liquidproducts which either freeze or become too viscous to pump under coldtemperature conditions, e.g., those encountered in underground wells inCanada and the Rocky Mountains. The need exists for solid cross-linkerswhich can be used to generate high, thermally stable viscosity in a lowand/or high pH environment, and which can be used under these coldtemperature conditions by pre-blending with the organic polymer, addedas a solid to an aqueous polymer solution or dissolved in water andadded to the aqueous polymer solution.

The need also exists for solid cross-linkers in off-shore fracturingoperations, where the weight of chemicals being shipped and stored iscritical. Solid cross-linkers, which contain two or more times theactive Ti or Zr content of their liquid counterparts are desired becausethey would allow fracturing operations to be completed moreeconomically.

The need also exists for solid cross-linkers that are non-flammable.Existing liquid solvent-based zirconate cross-linkers are flammableliquids. Even aqueous-based liquids generally comprise an organicco-solvent, and therefore are also flammable.

The present invention addresses these needs.

SUMMARY OF THE INVENTION

The present invention overcomes problems with liquid cross-linkingagents by providing a process to prepare a solid cross-linking agent.The solid cross-linking agent is produced by a process which comprises:(a) contacting a zirconium complex with an alkanolamine in an alcoholsolvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (b)adding water to the product of step (a) in an amount of 0.5 moles ofwater per mole of zirconium; (c) maintaining the product of step (b) fora sufficient period of time for the product to reach equilibrium; and(d) removing the solvent from the product of step (c) to form a solidzirconium cross-linking agent. In step (c), the temperature is generallyfrom about 25° C. to about 90° C., preferably from 50° C. to 80° C. Instep (d), the solvent is preferably removed by evaporation, for example,on a rotary evaporator. The temperature for evaporation should be belowthe melting or softening point of the zirconium cross-linking agent,preferably between 50° C. and 80° C.

There is further provided a cross-linking composition comprising (a) anaqueous liquid; (b) a buffer; (c) a cross-linkable organic polymer; and(d) a zirconium cross-linking agent, wherein the cross-linking agent isprepared by process comprising (1) contacting a zirconium complex withan alkanolamine in an alcohol solvent, wherein the mole ratio ofzirconium to alkanolamine is 1:1; (2) adding water to the product ofstep (1) in an amount of 0.5 moles of water per mole of zirconium; (3)maintaining the product of step (2) for a sufficient period of time forthe product to reach equilibrium; and (4) removing the solvent from theproduct of step (3) to form a solid zirconium cross-linking agent.

There is further provided methods for using the solid zirconiumcross-linking agent in a cross-linking composition in oil fieldapplications. Thus, there is provided a method for hydraulicallyfracturing a subterranean formation comprising introducing into asubterranean formation at a flow rate and pressure sufficient to create,reopen and/or extend a fracture in the formation, (a) an aqueous liquid;(b) a buffer; (c) a cross-linkable organic polymer; and (d) a zirconiumcross-linking agent, wherein the cross-linking agent is prepared byprocess comprising (1) contacting a zirconium complex with analkanolamine in an alcohol solvent, wherein the mole ratio ofalkanolamine to zirconium is 1:1; (2) adding water to the product ofstep (1) in an amount of 0.5 moles of water per mole of zirconium; (3)maintaining the product of step (2) for a sufficient period of time forthe product to reach equilibrium; and (4) removing the solvent from theproduct of step (3) to form a solid zirconium cross-linking agent. Thecomponents of the cross-linking composition can be introduced into theformation in any order or combination.

Optionally, the zirconium cross-linking agent may be dissolved in asolvent or solution (preferably water) containing optional components toprovide a zirconium solution. The cross-linkable organic polymer is alsopreferably contacted with the aqueous liquid to prepare a base gel. Thezirconium solution is introduced into the wellbore simultaneously withor sequentially after introducing the base gel into the wellbore. Fromthe wellbore, the combination of base gel and zirconium solution areintroduced into the formation.

This invention provides a method for plugging a permeable zone or leakin a subterranean formation which comprises introducing into said zoneor said leak, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkableorganic polymer; and (d) a zirconium cross-linking agent, wherein thecross-linking agent is prepared by process comprising (1) contacting azirconium complex with an alkanolamine in an alcohol solvent, whereinthe mole ratio of alkanolamine to zirconium is 1:1; (2) adding water tothe product of step (1) in an amount of 0.5 moles of water per mole ofzirconium; (3) maintaining the product of step (2) for a sufficientperiod of time for the product to reach equilibrium; and (4) removingthe solvent from the product of step (3) to form a solid zirconiumcross-linking agent.

DETAILED DESCRIPTION OF THE INVENTION

Trademarks and tradenames are shown herein in upper case.

This invention is a process which provides a solid zirconiumalkanolamine cross-linking agent at a particular mole ratio ofalkanolamine to zirconium and at a particular mole ratio of water tozirconium. Surprisingly, it has been found that solid zirconiumalkanolamine cross-linking agent is prepared according to the process ofthis invention that can be easily handled and overcomes thedisadvantages of known zirconium alkanolamine cross-linking agents thatare used as solutions.

The solid cross-linking agent is produced by a process which comprises:(a) contacting a zirconium complex with an alkanolamine at a particularmole ratio of alkanolamine to zirconium in an alcohol solvent; (b)hydrolyzing the product of step (a) by adding water; (c) maintaining theproduct of step (b) for a sufficient period of time for the product toreach equilibrium; and (d) removing the solvent from the product of step(c) to form a solid zirconium cross-linking agent. Preferably, water isadded in an amount of about 0.5 mole of water for each mole ofzirconium. Optionally, a diluent is added in step (c) or (d) to act asan anti-caking agent or to aid in flowability of the solid product.

While not wishing to be bound by theory, it is believed that the solidzirconium cross-linking agent produced comprises a mu oxo dimer ofzirconium with alkanolamine. A mu oxo dimer is a combination of twozirconium atoms bridged by an oxygen atom. The structure or structuresof such suspected mu oxo dimer has not been determined. The solidcross-linking agent may comprise other products, many of which willdepend on reaction conditions.

A cross-linking composition based on the solid cross-linking agentprepared in this invention can be prepared by (1) blending thecross-linking agent with a cross-linkable organic polymer, then addingthe blend as a solid to an aqueous liquid; or (2) dissolving thecross-linking agent in water, and adding the solution thus prepared toan aqueous liquid. In the latter alternative, the polymer may bepre-dissolved in the aqueous liquid or may be added to the aqueousliquid after the cross-linking agent has been added.

A solid zirconium cross-linking agent is prepared by a process whichcomprises contacting a zirconium complex with an alkanolamine in analcohol solvent. The mole ratio of alkanolamine to zirconium is 1:1.Surprisingly, outside of these ranges, for example, at a molar ratio of2 moles of alkanolamine to zirconium, a solid product does not form.

A number of tetraalkyl zirconates (also known as zirconiumtetraalkoxides) can be used as the zirconium complex to prepare theabove zirconium cross-linking composition, e.g., tetra-isopropylzirconate, tetra-n-propyl zirconate, and tetra-n-butyl zirconate. Thepreferred tetraalkyl zirconate is tetra-n-propyl zirconate, available asTYZOR NPZ organic zirconate, a solution in n-propanol, with a zirconiumcontent as ZrO₂ of about 28% by weight, and available from E. I. du Pontde Nemours and Company, Wilmington, Del.

Examples of suitable alkanolamines include, but are not limited to,triethanolamine, tri-n-propanolamine, tri-isopropanolamine, anddiisopropanolamine. Preferably, the alkanolamine is triethanolamine.

Contacting the above tetraalkyl zirconates with the alkanolamine can becarried out at a variety of temperatures, e.g., between 25° C. and 90°C., preferably between 50° C. and 80° C., and in any order. The mixtureis then maintained at this temperature for a sufficient period to reachequilibrium. For example, about 2 hours at 60° C. is adequate, but otherperiods may also be used.

The product of step (a) is then reacted with water in step (b) at atemperature between 25° C. and 90° C., preferably between 50° C. and 80°C., to form a hydrolysate. The amount of water should preferably be 0.5mole of water per mole of zirconium to maximize the yield of desirableproduct, including the suspected mu oxo dimer of zirconium andalkanolamine. Surprisingly, outside of this ratio, for example, at amolar ratio of 2 moles of water per mole of zirconium, a solid productdoes not form.

Preferably, in step (b), water is added slowly as a dilute solution inan alcohol to minimize localized overreaction areas. For ease of alcoholrecovery, it is preferred that the same alcohol be used as present inthe zirconium complex. For example, if the zirconium complex is TYZORNPZ organic zirconate, a dilute solution of water in n-propanol is used.By “dilute solution” is meant a solution of water in alcohol up to about10% water in the alcohol. A solution of about 9-10% water in the alcoholis adequate dilution, but other concentrations may also be adequate.

In step (c), the product of step (b) is maintained at conditions for asufficient period of time for the product to reach equilibrium.Equilibrium is reached after (1) the zirconium complex produced in step(a) has reacted with the water to form an intermediate product and (2)this intermediate product reacts with more product of step (a), as isproposed above, to form a mu-oxo dimer product. Time will depend onreaction conditions, such as temperature, pressure, mass transfer rates(e.g., depending on intensity of agitation). Thus, the process does notproceed to step (d) until the equilibrium is established. This step (c)can be carried out at a variety of temperatures, e.g., between 25° C.and 90° C., preferably between 50° C. and 80° C., and is then generallyheld at this temperature for a sufficient period to reach equilibrium.It has been found that holding the composition in step (c) at atemperature of 60° C. for about 6 hours is adequate, but other periodsand temperatures may also be used.

In step (d), solvent is removed from the product of step (c), forexample by distilling under vacuum, to remove the alcohol. Preferably,the distillation is carried out in a rotary evaporator, graining bowl,or other vacuum device with rotating parts to keep the solid in motionas it solidifies to limit and/or prevent lump formation. Spray dryingmay also be used in step (d) to remove the solvent. Many types ofsuitable equipment may be used, as are well known to those skilled inthe art. The solution may also be simply dried in air, although forenvironmental reasons this is not a method of choice. The temperature ofdrying should be below the melting or softening point of the zirconiumcomplex. Preferably, the temperature is between 50° C. and 80° C.

Preferably, a diluent is added to the solid cross-linking agent. Adiluent may be added to the product of step (c) or to the solid afterstep (d). The diluent can be any material which may enhance propertiesof the solid, and does not adversely affect the solid or its use as acrosslinking agent. A diluent is added for such purposes as to help inthe crystallization step, increase the crystallinity of the solidparticles, provide an anti-caking action, improve the flowability of thesolid material, enable easier solution of the solids in water, improveproduct stability, or for other purposes.

For example, the diluent may be a salt or metal oxide. The diluent maybe selected from the group consisting of potassium chloride,tetramethylammonium hydroxide, titanium dioxide and silicon dioxide.Potassium chloride and tetramethylammonium hydroxide, for example, makethe solid easier to remove from the drying unit than undiluted solids.The resulting solids are also more free-flowing and hence much moreeasily handled in the field. Insoluble inert compounds such as titaniumdioxide and silicon dioxide may also be used to improve handling, andmay be added before or after the drying step (d).

The solid zirconium cross-linking agent has a high concentration ofzirconium relative to conventional cross-linking agents which aregenerally provided as solutions in water, alcohol or mixtures thereof.The zirconium content may be, for example, from 15 to 40% by weightbased on the total weight of the cross-linking agent. Typical solutionsof cross-linking agents comprise less than 10% zirconium by weight basedon the total weight of the cross-linking agent.

The present invention also provides a cross-linking composition whichcomprises (a) an aqueous liquid; (b) a pH buffer; (c) a cross-linkableorganic polymer; and (d) a zirconium cross-linking agent as preparedaccording to the process described above. Optionally, the compositionmay comprise a solvent.

The aqueous liquid (a) is typically selected from the group consistingof water, aqueous alcohol, and aqueous solution of a clay stabilizer.The alcohol is methanol or ethanol. Clay stabilizers include, forexample, hydrochloric acid and chloride salts, such as,tetramethylammonium chloride (TMAC) or potassium chloride. Aqueoussolutions comprising clay stabilizers may comprise, for example, 0.05 to0.5 weight % of the stabilizer, based on the total weight of thecross-linking composition. Preferably, the clay stabilizer istetramethylammonium chloride or potassium chloride. Preferably, theaqueous liquid is water, aqueous methanol, aqueous ethanol, an aqueoussolution of potassium chloride, or a combination of two or more thereof.

The cross-linking composition comprises an effective amount of a pHbuffer (b) to control pH. The pH buffer may be acidic, neutral or basic.The pH buffer is generally capable of controlling the pH from about pH 3to about pH 12. For example, in a composition for use at pH of about4-5, an acetic acid-based buffer can be used. In a composition for useat a pH of 5-7, a fumaric acid-based buffer or a sodium diacetate-basedbuffer can be used. In a composition for use at a pH of 7-8.5, a sodiumbicarbonate-based buffer can be used. In a composition for use at a pHof 9-12, a sodium carbonate or sodium hydroxide-based buffer can beused. Other suitable pH buffers can be used, as are known to thoseskilled in the art.

The composition further comprises a cross-linkable organic polymer (c).Suitable cross-linkable organic polymers are selected from the groupconsisting of solvatable polysaccharides, polyacrylamides andpolymethacrylamides. Preferably, the organic polymer is a solvatablepolysaccharide and is selected from the group consisting of gums, gumderivatives and cellulose derivatives. Gums include guar gum and locustbean gum, as well as other galactomannan and glucomannan gums, such asthose derived from sennas, Brazilwood, tera, honey locust, karaya gumand the like. Gum derivatives include hydroxyethylguar (HEG),hydroxypropylguar (HPG), carboxyethylhydroxyethylguar (CEHEG),carboxymethylhydroxypropylguar (CMHPG), carboxymethyl guar (CMG), andthe like. Cellulose derivatives include those containing carboxylgroups, such as carboxymethylcellulose (CMC),carboxymethylhydroxy-ethylcellulose (CMHEC), and the like. Thesolvatable polysaccharides can be used individually or in combination;usually, however, a single material is used. Guar derivatives andcellulose derivatives are preferred, such as, HPG, CMC and CMHPG. HPG isgenerally more preferred based upon its commercial availability anddesirable properties. However, CMC and CMHPG may be more preferred incross-linking compositions when the pH of the composition is less than6.0 or higher than 9.0, or when the permeability of the formation issuch that one wishes to keep the residual solids at a low level toprevent damage to the formation.

The zirconium cross-linking agent (d) is the solid zirconium productprepared as described previously, and made by a method comprising: (a)contacting a zirconium complex with an alkanolamine in an alcoholsolvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (b)adding water to the product of step (a) in an amount of 0.5 moles ofwater per mole of zirconium; (c) maintaining the product of step (b) fora sufficient period of time for the product to reach equilibrium; and(d) removing the solvent from the product of step (c) to form a solidzirconium cross-linking agent.

The zirconium cross-linking agent of the invention can be added as asolid or optionally dissolved in solution (preferably water). The solidcross-linking agent can be pre-blended with the solid polymer prior todissolving both the solid cross-linking agent and solid polymer in theaqueous liquid. Alternatively, the solid zirconium cross-linking agentcan be added to the aqueous liquid as a solid or in solution, before orafter addition of the polymer. Typically, the cross-linkable polymer ismixed with the aqueous liquid such as water or mixed water/organicsolvent or with an aqueous solution to form a base gel prior to addingthe zirconium cross-linking agent. Organic solvents that may be usedinclude alcohols, glycols, polyols, and hydrocarbons such as diesel.

The composition may comprise optional components, including those whichare common additives for oil field applications. Thus, the compositionmay further comprise one or more of proppants, friction reducers,bactericides, hydrocarbons, chemical breakers, polymer stabilizers,surfactants, formation control agents, and the like. Proppants includesand, bauxite, glass beads, nylon pellets, aluminum pellets and similarmaterials. Friction reducers include polyacrylamides. Hydrocarbonsinclude diesel oil. Chemical breakers break the cross-linked polymer(gel) in a controlled manner and include enzymes, alkali metalpersulfate, and ammonium persulfate. Polymer stabilizers includemethanol, alkali metal thiosulfate, and ammonium thiosulfate.

These optional components are added in an effective amount sufficient toachieve the desired cross-linking performance based on the individualcomponents, desired cross-linking time, temperature and other conditionspresent in the formation being fractured or permeable zone beingplugged.

The cross-linking composition is produced by mixing the solid orsolution of zirconium cross-linking agent with the other components, inany order. A solution of zirconium cross-linking agent (zirconiumsolution) can be prepared by dissolving the solid zirconiumcross-linking agent in an appropriate solvent or solution containingother components. Suitable solvents include water, water-alcoholmixtures, and alcohol wherein alcohol is methanol or ethanol.Preferably, the solvent is water. A solution containing other componentsmay be an aqueous solution or an alcohol solution. Other components mayinclude, for example, buffer, clay stabilizers, other optionalcomponents.

For example, in one particular application in an oil field, an aqueoussolution of the solid zirconium cross-linking agent and optionalcomponents are introduced into a formation, while the cross-linkableorganic polymer and aqueous liquid are introduced into the formation asa combined separate stream. Alternatively, all components may bepremixed and introduced into a subterranean formation as a singlestream. Advantageously, the components may be mixed in differentcombinations, and more advantageously, the components may be mixed justprior to use to enable easy variation and adjustment of thecross-linking rate.

This invention provides a method for hydraulically fracturing asubterranean formation, which comprises introducing into the formationat a flow rate and pressure sufficient to create, reopen, and/or extendone or more fractures in the formation, a cross-linking compositionwhich comprises (a) an aqueous liquid; (b) a pH buffer; (c) across-linkable organic polymer; and (d) a solid zirconium cross-linkingagent wherein the cross-linking agent is prepared by a processcomprising: (1) contacting a zirconium complex with an alkanolamine inan alcohol solvent, wherein the mole ratio of alkanolamine to zirconiumis 1:1; (2) adding water to the product of step (1) in an amount of 0.5moles of water per mole of zirconium; (3) maintaining the product ofstep (2) for a sufficient period of time for the product to reachequilibrium; and (4) removing the solvent from the product of step (3)to form a solid zirconium cross-linking agent.

In one embodiment of the method for hydraulically fracturing asubterranean formation, a solution of solid zirconium cross-linkingagent and cross-linkable polymer are contacted prior to theirintroduction into the formation, such that the cross-linking agent andpolymer react to form a cross-linked gel. The cross-linked gel is thenintroduced into the formation at a flow rate and pressure sufficient tocreate, reopen, and/or extend a fracture in the formation. In thismethod, a base gel is prepared by mixing a cross-linkable organicpolymer with an aqueous liquid. A cross-linked gel is prepared by mixingthe base gel with a solution of the solid zirconium cross-linking agent,wherein the cross-linking agent is prepared by a process comprising: (a)contacting a zirconium complex with an alkanolamine in an alcoholsolvent, wherein the mole ratio of alkanolamine to zirconium is 1:1; (b)adding water to the product of step (a) in an amount of 0.5 moles ofwater per mole of zirconium; (c) maintaining the product of step (b) fora sufficient period of time for the product to reach equilibrium; and(d) removing the solvent from the product of step (c) to form a solidzirconium cross-linking agent. The solution of the solid zirconiumcross-linking agent is preferably prepared by dissolving the solid agentin water. At least one of the solution of zirconium cross-linking agentor the base gel further comprises a pH buffer.

Alternatively, the subterranean formation may be penetrated by awellbore, such that contacting the solution of the solid zirconiumcross-linking agent with the base gel occurs in the wellbore and thecross-linked gel is introduced into the formation from the wellbore.This method of hydraulically fracturing a subterranean formationpenetrated by a wellbore comprises (a) preparing a base gel by mixing across-linkable organic polymer with an aqueous liquid; (c) introducingthe base gel into the wellbore; (d) simultaneously with or sequentiallyafter introducing the base gel into the wellbore, introducing a solutionof the solid zirconium cross-linking agent; (e) permitting the base geland the solution of zirconium cross-linking agent to react to form across-linked gel; and (f) introducing the cross-linked gel into theformation from the wellbore at a flow rate and pressure sufficient tocreate, reopen, and/or extend a fracture in the formation. A pH bufferis independently admixed with the base gel, the zirconium or both priorto introducing the base gel and the zirconium solution into thewellbore.

Upon creation of a fracture or fractures, the method may furthercomprise introducing a cross-linking composition comprising thezirconium solution, a cross-linkable organic polymer and proppant intothe fracture or fractures. This second introduction of the zirconiumsolution is preferably performed in the event the cross-linkingcomposition used to create the fracture or fractures did not compriseproppant.

Another use for the zirconium cross-linking agent of the presentinvention relates to a method for selectively plugging permeable zonesand leaks in subterranean formations, which comprises introducing intothe permeable zone or the site of the subterranean leak, a cross-linkingcomposition comprising (a) an aqueous liquid; (b) a pH buffer; (c) across-linkable organic polymer; and (d) an aqueous solution of a solidzirconium cross-linking agent as described previously. The pH buffer maybe admixed with the solution of zirconium cross-linking agent prior tointroducing the cross-linking composition into the permeable zone orsite of the leak.

In a first embodiment of the method for plugging a permeable zone or aleak in a subterranean formation, the aqueous liquid, pH buffer,cross-linkable organic polymer and the zirconium solution are contactedprior to their introduction into the subterranean formation, such thatthe polymer and zirconium cross-linking agent react to form across-linked aqueous gel, which gel is then introduced into theformation.

In an alternative embodiment of the method for plugging a permeable zoneor a leak in a subterranean formation, the zirconium solution and thecross-linkable organic polymer are introduced separately, eithersimultaneously or sequentially, into the permeable zone or the site ofthe subterranean leak such that cross-linking occurs within thesubterranean formation. This method comprises (a) preparing a base gelby mixing a cross-linkable organic polymer with an aqueous liquid; (b)introducing the base gel into the into the permeable zone or the site ofthe subterranean leak, (d) simultaneously with or sequentially after,introducing the base gel into the into the permeable zone or the site ofthe subterranean leak, introducing the zirconium solution into thepermeable zone or the site of the subterranean leak; (e) permitting thebase gel and the cross-linking agent to react to form a cross-linkedaqueous gel to plug the zone and/or leak. At least one of the solutionof zirconium cross-linking agent or the base gel further comprises a pHbuffer.

The relative amounts of cross-linkable organic polymer and the zirconiumcomplex may vary. One uses small but effective amounts which for bothwill vary with the conditions, e.g., the type of subterranean formation,the depth at which the method (e.g., fluid fracturing, permeable zoneplugging or leak plugging) is to be performed, temperature, pH, etc.Generally, one uses as small an amount of each component as will providethe viscosity level necessary to effect the desired result, i.e.,fracturing of the subterranean formation, or plugging permeable zones orleaks to the extent necessary to promote adequate recovery of oil or gasfrom the formation.

For example, satisfactory gels can generally be made for fluidfracturing by using the cross-linkable organic polymer in amounts up toabout 1.2 weight % and the cross-linking composition in amounts up toabout 0.50 weight % of the zirconium cross-linking agent, withpercentages being based on the total weight. Preferably, from about 0.25to about 0.75 weight % of the cross-linkable organic polymer is used andfrom about 0.05 to about 0.25 weight % of the zirconium cross-linkingagent is used.

In a method for plugging permeable zones or leaks, generally about 0.25to 1.2 weight % of a cross-linkable organic polymer is used, preferably0.40 to 0.75 weight %, based on the total weight. Generally, about 0.01to 0.50 weight % of the zirconium cross-linking agent is used,preferably 0.05 to 0.25 weight %, based on the total weight.

The amount of zirconium cross-linking agent used to cross-link theorganic polymer is that which provides a zirconium ion concentration ina range from about 0.0005 weight % to about 0.1 weight %, based on thetotal weight. The preferred concentration of zirconium ion is in therange of from about 0.001-0.05 weight %, based on the total weight.

Typically, the solution of zirconium cross-linking agent of thisinvention can be used at a pH of from about 3 to 11. For low temperatureapplications (150-250° F., 66-121° C.), carbon dioxide-based energizedfluids may be used. In this case, a pH for the cross-linking compositionof about 3 to about 6 is preferred. For moderate or high temperatureapplications (250-400° F., 121-204° C.), a pH of about 9 to about 11 ispreferred. Advantageously, the solution of zirconium complex of thisinvention is used at a temperature of 250-300° F. (121-149° C.).

EXAMPLES

The preparation of the compositions in the Examples and in the Controlswere each carried out in closed vessels containing an agitator,thermometer, condenser, nitrogen inlet and dropping funnel. Unlessspecified otherwise, percentages are given by weight. Temperatures aregiven in degrees Celsius. The cross-linking properties of thecompositions of this invention are given in the Examples as a functionof the viscosity of carboxymethylhydroxypropylguar cross-linked with thezirconate of this invention.

Preparation of Base Gel

A Waring blender jar was filled with 1 liter of distilled water. To thiswas added 2 g of a 50% aqueous solution of tetramethylammonium chlorideclay stabilizer. Agitation was started and 3.6 g ofcarboxymethylhydroxypropylguar (CMHPG) was sprinkled into the vortex ofthe agitating solution. The pH of the resultant slurry was adjusted to 6with sodium diacetate and agitation continued for 30 minutes. The pH wasthen adjusted to 10.3 with 10% sodium hydroxide solution. Agitation wasstopped and the gel was allowed to stand for 30 minutes or more beforeuse.

Viscosity Measurement of Zirconate Cross-Linked Base Gel

To 250 ml of a vigorously agitated sample of base gel in a Waringblender jar, was added 0.00032 moles of zirconium (0.2-1.0 ml dependingon percent zirconium in the solid cross-linking agent—hereinafterreferred to as the Standard Loading Density). Agitation was continuedfor about 15-180 seconds. A 25 ml sample of the cross-linker containinggel was placed in the cup of the FANN 50 Viscometer with an R-1, B-3configuration and viscosity was measured at 275° F. (135° C.) and 122rpm at 100 reciprocal seconds of shear.

Comparative Example A

Triethanolamine (135.2 g) was added to 100 g of tetra-n-propyl zirconatesolution (TYZOR NPZ organic zirconate, available from E. I. du Pont deNemours and Company, Wilmington, Del.). The reaction mixture was heatedto 60° C. and held there for 4 hours. Upon completion of the reactionthe resultant solution of tetra(triethanolamine) zirconate wasconcentrated on a rotary evaporator under reduced pressure to yield 155g of a viscous yellow oil, which contained 13.2% zirconium. Thismaterial was placed in a 32° F. (0° C.) freezer, which resulted in thematerial setting up as an immobile gummy solid.

This example shows that simply removing solvents from a solvent-basedzirconate complex does not yield a solid that can be readily and easilyhandled for measuring and/or metering into a cross-linking composition.

Comparative Example B

This Comparative Example illustrates that preparing a zirconiumtriethanolamine cross-linking agent, having a mole ratio oftriethanolamine to zirconium of 2:1, according to the process of thisinvention does not produce a solid cross-linking agent.

A 500-ml flask equipped with agitator, reflux condenser, N₂ source andagitator was charged with 100 g (0.227 moles) of tetra-n-propylzirconate solution (TYZOR NPZ organic zirconate). Agitation was startedand 67.6 g (0.454 moles) of triethanolamine were added dropwise. Thereaction mixture was heated to 60° C. and held at this temperature for 2hours. Then, a mixture glycerol, 20.9 g (0.227 moles), and water, 4.1 g(0.227 moles) was added dropwise to the reaction mixture to provide asolution. The solution was held at 60° C. for an additional 2 hours, andthen the solvent removed on a rotary evaporator to give 124.8 g of aviscous yellow oil.

Comparative Example C

This Comparative Example illustrates that preparing a zirconiumtriethanolamine cross-linking agent, having a mole ratio oftriethanolamine to zirconium of 8:1, but without water, according to theprocess of this invention does not produce a solid cross-linking agent.

A 500-ml flask equipped with agitator, reflux condenser, N₂ source andagitator was charged with 100 g (0.227 moles) of tetra-n-propylzirconate solution (TYZOR NPZ organic zirconate). Agitation was startedand 270.3 g (1.816 moles) of triethanolamine were added dropwise. Thereaction mixture was heated to 60° C. and held at this temperature for 2hours. Then, the solvent removed on a rotary evaporator to give 324.6 gof a viscous yellow oil.

Comparative Example D

This Comparative Example illustrates that preparing a zirconiumtriethanolamine cross-linking agent, having a mole ratio oftriethanolamine to zirconium of 5:1 and large volume of water, accordingto the process of this invention does not produce a solid cross-linkingagent.

A 1000-ml flask equipped with agitator, reflux condenser, N₂ source andagitator was charged with 10.0 ml (0.0227 moles) of tetra-n-propylzirconate solution (TYZOR NPZ organic zirconate). Agitation was startedand 20.0 ml (0.122 moles) of triethanolamine were added dropwise. Thereaction mixture was heated to 60° C. and held at this temperature for 2hours. Then, a mixture of 30.0 ml of n-propanol and 540 ml of water wasadded. The solvent was removed on a rotary evaporator to give 24.2 g ofa viscous yellow oil.

Comparative Examples A through D show that simply removing solvent froma solvent-based zirconium complex, which can be used as a cross-linkingagent, as described in the patent literature, does not yield a solidthat can be readily and easily handled for measuring and/or meteringinto a cross-linking composition. Comparative Example A was a gummysolid. Comparative Examples B through D were viscous oils.

Comparative Example E

This Comparative Example illustrates that preparing a zirconiumtriethanolamine cross-linking agent, having a mole ratio oftriethanolamine to zirconium of 2:1, according to the process of thisinvention does not produce a solid cross-linking agent.

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 176.2 g (0.400 moles) of TYZOR NPZorganic zirconate. Agitation was started and 119.2 g (0.801 moles) oftriethanolamine were added. The resultant solution was held at 60° C.for two hours and then a solution of 3.6 g (0.20 moles) of water in 80ml of n-propanol was added dropwise. The resultant solution was held at60° C. an additional 6 hours and then placed on a rotary evaporator andthe solvent removed under vacuum to give a viscous yellow oil.

This example shows that use more than a 1:1 molar ratio of alkanolamineto zirconium does not give a solid product on evaporation of solvent.

Comparative Example F

This Comparative Example illustrates that preparing a zirconiumtriethanolamine cross-linking agent, having a mole ratio oftriethanolamine to zirconium of 2:1, according to the process of thisinvention does not produce a solid cross-linking agent.

A 500-ml flask, equipped with a thermocouple, dropping funnel, N₂ bleedand condenser was charged with 176.2 g (0.4 moles) of TYZOR NPZ organiczirconate. Agitation was started and 59.6 g (0.400 moles) oftriethanolamine were added. The resultant solution was held at 60° C.for two hours and then a solution of 7.2 g (0.40 moles) of water in 80ml of n-propanol was added dropwise. The resultant solution was held at60° C. an additional 6 hours and then placed on a rotary evaporator andthe solvent removed under vacuum to give a viscous yellow oil.

This example shows that use of more than a 0.5:1 molar ratio of water tozirconium alkanolamine complex does not give a solid product onevaporation of solvent.

Example 1

A 1000-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 352.4 g of TYZOR NPZ organic zirconate.Agitation was started and 119.2 g of triethanolamine were added. Theresultant solution was held at 60° C. for two hours and then a solutionof 7.2 g of water in 80 ml of n-propanol was added dropwise. Theresultant solution was held at 60° C. an additional 6 hours and thenplaced on a rotary evaporator and the solvent removed under vacuum togive 196 g of a pale yellow solid containing 37.2% Zr.

Example 2

A 500-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 352.4 g of TYZOR NPZ organic zirconate.Agitation was started and 152.8 g of tri-isopropanolamine were added.The resultant solution was held at 60° C. for two hours and then asolution of 7.2 g of water in 80 ml of n-propanol was added dropwise.The resultant solution was held at 60° C. an additional 6 hours and thenplaced on a rotary evaporator and the solvent removed under vacuum togive 230 g of a pale yellow solid containing 31.7% Zr.

Example 3

A 1000-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 176.2 g of TYZOR NPZ organic zirconate.Agitation was started and 59.6 g of triethanolamine were added. Theresultant solution was held at 60° C. for two hours and then a solutionof 3.9 g of water in 40 ml of n-propanol was added dropwise. Theresulting solution was held at 60° C. an additional 4 hours. Thisresulting solution was transferred to a 1000-ml pear shaped flask, 49 gof potassium chloride added and the solvent removed under vacuum on arotary evaporator to give 196 g of a pale yellow solid containing 18.6%Zr. The resultant solid was much more easily removed from the rotaryevaporator flask and was more granular in nature, i.e., easier tohandle.

Example 4

A 1000-ml flask, equipped with a thermocouple, dropping funnel, N2 bleedand condenser was charged with 176.2 g of TYZOR NPZ organic zirconate.Agitation was started and 59.6 g of triethanolamine were added. Theresultant solution was held at 60° C. for two hours and then a solutionof 3.9 g of water in 40 ml of n-propanol was added dropwise. Theresultant solution was held at 60° C. an additional 4 hours. Theresultant solution was transferred to a 1000-ml pear shaped flask, 49 gof tetramethylammonium chloride added and the solvent removed undervacuum on a rotary evaporator to give 196 g of a pale yellow solidcontaining 18.6% Zr. The resulting solid was much more easily removedfrom the rotary evaporator flask and was more granular in nature, i.e.,easier to handle. Results

Table 1 below shows the performance of a 30 lb/1000 gallon (3600 g/1000liters) CMHPG gel cross-linked with the zirconium products of theExamples. In this table, “Fann Time Max, min.” means the time, inminutes, it takes to reach maximum viscosity. The viscosity at thismaximum time is labeled “Cp @ Max.”, to indicate viscosity in centipoise(Cp) and the viscosity after 90 minutes at the test temperature of 275°F. (135° C.) is labeled “Cp @ 90 min.”. The solid zirconatecross-linking agent was added to the pre-formed polymer base gel as asolid. Comparative Example A was warmed to lower viscosity and added asa viscous liquid.

TABLE 1 Example Performance Alkanolamine:Zr, Water:Zr, Fann Time Cp @ Cp@ 90 Example % Zr Alkanolamine mole ratio mole ratio Max, min Max min.Comparative A 13.2 Triethanolamine 4 1.5 1125 680 1 37.2Isopropanolamine 1 0.5 3.5 1325 530 2 31.7 Triethanolamine 1 0.5 3.0 820120 3 18.6 Triethanolamine 1 0.5 3.0 1200 622 4 18.6 Triethanolamine 10.5 2.5 1400 528

Table 1 shows that commercially available, cross-linking agent used inComparative Example A generates and maintains good viscosity. However,when placed in a freezer, the material became an immobile gum, whichwould be very difficult to handle in colder climates.

The solid cross-linking agents of Examples 1 and 2, generate excellentviscosity at a desirable rate of cross-linking and, because they aresolids, do not lose effectiveness in colder climates.

The addition of a diluent, such as potassium chloride ortetramethylammonium chloride improved granularity and flowability of thesolid zirconate cross-linking agents of Examples 3 and 4, whilemaintaining good viscosity performance.

1. A method for hydraulically fracturing a subterranean formationcomprising introducing into a subterranean formation at a flow rate andpressure sufficient to create, reopen and/or extend a fracture in theformation, (a) an aqueous liquid; (b) a buffer; (c) a cross-linkableorganic polymer; and (d) a solid zirconium cross-linking agent having azirconium content of from 15% to 40% based on the total weight of thecrosslinking agent.
 2. The method of claim 1 wherein the aqueous liquid,the buffer, the cross-linkable organic polymer, and the solid zirconiumcross-linking agent are premixed and introduced into the subterraneanformation as a single stream.
 3. The method of claim 1 wherein thesubterranean formation is penetrated by a wellbore; the cross-linkableorganic polymer and aqueous liquid are contacted to form a base gel; thesold zirconium cross-linking agent is dissolved in a solvent or solutioncontaining other components to provide a zirconium solution; and whereinthe method comprises introducing the base gel into the wellbore and/orformation; introducing the zirconium solution into the wellbore and/orformation simultaneously with or sequentially after introducing the basegel into the wellbore and/or formation; and permitting the base gel andthe zirconium solution to react to form a cross-linked gel in situwithin the wellbore and/or formation.
 4. The method of claim 3 whereinthe solid zirconium cross-linking agent is dissolved in water.
 5. Themethod of claim 3 further comprising introducing a second compositioncomprising a zirconium solution, a cross-linkable organic polymer, and aproppant into the wellbore and/or formation.
 6. The method of claim 3wherein the zirconium solution, the base gel or both further comprise aproppant.
 7. The method of claim 3 wherein the solid-zirconiumcross-linking agent is used at a pH of from about 3 to
 11. 8. The methodof claim 1 wherein the subterranean formation has a temperature of fromabout 150° F. to about 250° F.
 9. The method of claim 8 furthercomprising introducing to the subterranean formation carbondioxide-based energized fluid.
 10. The method of claim 3 wherein thesolid zirconium cross linking agent is present in an amount sufficientto provide a zirconium ion concentration of from about 0.0005 wt. % toabout 0.1 wt. % based on the total weight of the crosslinked gel.
 11. Amethod for plugging a permeable zone or leak in a subterranean formationcomprising introducing into said zone or said leak, (a) an aqueousliquid; (b) a buffer; (c) a cross-linkable organic polymer; and (d) asolution of a solid cross-linking agent comprising azirconium-alkanolamine complex having a zirconium content of from 15% to40% based on the total weight of the crosslinking agent.
 12. The methodof claim 11 wherein the aqueous liquid, pH buffer, cross-linkableorganic polymer; and solid cross-linking agent comprising azirconium-alkanolamine complex are contacted prior to their introductiona permeable zone or leak zone in the subterranean formation.
 13. Themethod of claim 11 wherein the subterranean formation is penetrated by awellbore; the solid cross-linking agent comprising azirconium-alkanolamine complex is dissolved in a solvent or solutioncontaining other components to provide a zirconium solution; andcontacting of the zirconium solution with the aqueous liquid and thecross-linkable organic polymer occurs in the wellbore and/or formation.14. The method of claim 13 wherein the solid cross-linking agentcomprising a zirconium-alkanolamine complex is dissolved in water. 15.The method of claim 13 wherein the zirconium solution and cross-linkablepolymer are introduced sequentially to the wellbore and/or formation.16. The method of claim 13 wherein the zirconium solution andcross-linkable polymer are introduced simultaneously to the wellboreand/or formation.
 17. The method of claim 13 wherein the zirconiumsolution, the base gel or both further comprise a proppant.
 18. Themethod of claim 13 wherein the solid-zirconium cross-linking agentcomprising a zirconium-alkanolamine complex is used at a pH of fromabout 3 to
 11. 19. The method of claim 13 wherein the subterraneanformation has a temperature of from about 150° F. to about 250° F.
 20. Amethod for hydraulically fracturing a subterranean formation comprisingintroducing into a subterranean formation at a flow rate and pressuresufficient to create, reopen and/or extend a fracture in the formation,(a) an aqueous liquid; (b) a buffer; (c) a cross-linkable organicpolymer; and (d) a zirconium cross-linking agent, wherein thecross-linking agent is prepared by process comprising (i) contacting azirconium complex with an alkanolamine in an alcohol solvent, whereinthe zirconium complex is a tetraalkyl zirconate, wherein thealkanolamine is selected from the group consisting of triethanolamine,tri-n-propanolamine, tri-isopropanolamine, and diisopropanolamine, andwherein the mole ratio of alkanolamine to zirconium is 1:1; (ii) addingwater to the product of step (i) in an amount of 0.5 moles of water permole of zirconium; (iii) maintaining the product of step (ii) for asufficient period of time for the product to reach equilibrium; and (iv)removing the solvent from the product of step (iii) to form a solidzirconium cross-linking agent.